In cellular radiotelephone communication systems, call handoffs are often required in order to maintain communication with mobiles as they move within the system. Handoff is the process whereby an in progress call is transferred from one RF coverage area (cell) to another, in coordination with a mobile's movements. It is also the process whereby a call is transferred to another channel within a cell because of interference within the cell. The process of handing off a call is one of the most delicately balanced functions relating to cellular radiotelephone systems and service because of the high level of synchronization required among various system processing elements.
In order to effectuate successful handoffs, current analog cellular systems continuously monitor the quality of every call throughout the system. The system must recognize when the quality of a call falls below a predetermined threshold and must then determine what other neighboring cell can most satisfactorily provide continued service. Once a more suitable cell is identified, the system sends instructions to the mobile, directing it to tune to a channel within the newly identified cell. The mobile confirms that it is leaving its current channel, tunes to the new channel, synchronizes to the new channel and begins transmitting.
In digital cellular radiotelephone systems, the procedure is modified somewhat in that the mobile measures channels in other cells as well as its current serving channel and reports these measurements, consisting of signal strength, back to the system. Since, the mobile cannot determine whether its measurements are as a result of interference, digital systems typically employ scan receivers to determine when a handoff, power change, or antenna change is required. This is done by measuring the mobile's signal strength as it moves within its currently serving cell. These measurements are taken by both the serving cell and adjacent cell scan receivers. Thereafter, the mobile's absolute signal strength is used to determine when a power change is required. The relative signal strength among the base site's antennas determines when an antenna change is required, and the relative signal strength between base sites is used to determine when a handoff is required.
Unlike the analog counterpart, digital mobiles do not continuously transmit. Thus, in a TDMA system, characterized by the shared allocation of resources, it is necessary for adjacent cell scan receivers to know when the candidate mobile is transmitting in order to make accurate signal strength measurements. If all cells within the TDMA system are synchronized, adjacent cell scan receivers can readily determine when to make such measurements based upon adjacent cell local timing.
By synchronized, it is meant that the timing within each cell is substantially identical. In the case of time-shared channels, where the information is organized into blocks or frames of some uniform number of symbols, synchronization pertains to frame synchronization. Frame synchronization is equivalent to being able to generate a square wave at a frame rate, with the zero crossings coincident with the transitions from one frame to the next.
Unfortunately, synchronization within any communication system implies cost to the system. The most obvious cost is in the need for additional hardware and/or software required for data acquisition. Less obvious costs lie in the extra time required to achieve synchronization prior to communications. In light of these factors, the typical TDMA communication system is not synchronized and therefore susceptible to various timing errors and delays.
During normal radiotelephone communications, these timing anomalies typically go unnoticed. During handoffs, however, even minor timing errors or delay may result in the failure to hand a call off at the proper time. This type failure is readily recognizable to the system subscriber and is typically characterized by reduced call quality, interference with neighboring coverage areas or even the undesired termination of the call.
It would be extremely advantageous, therefore, to provide within a TDMA communication system, a method for determining frame synchronization.